Fastening the Fan

My engine fan vibrated loose three times! I used locktite (as in the manual) but that seems insufficient. If this bolt vibrates loose, it will push up the clutch against the clutch bell, effectively blocking the clutch completely. Luckily, this seems to happen especially when starting the engine and not when the engine is running already.
If the clutch has been blocked, you will notice because the engine cannot be turned anymore, neither by the starting engine nor by hand. DO NOT try to force it rotating by hand!! This may damage the ball bearings, either in your engine or in your frame. My already-weak bearing (see discussion above on rusting ball bearings) was possibly damaged by me because of this.
I asked on the club and someone suggested to use a ring between the bolt and the fan. Furthermore I tightened the bolt very tight (about 10Nm?). Even this combination of ring and tight fastening showed insufficient, only if you use loctite in the bolt it may stay in place.
Looking back, I always cleaned but did not degrease the stuff before loctiting. This may have caused suboptimal fastening of the loctite. Now I always degrease before applying loctite and CA, and I have not had this problem anymore. Once I even put a little bit of CA in the nut, just to be sure. CA on metal fits extremely tight! More recently I use standard blue, or sometimes a little green loctite, fits OK as long as you degrease.

Insert such ring between the engine bolt and the fan, to prevent vibrating the bolt loose.